Generalize documents by using "API provider" rather than "Telco Operator"

[tanjadegroot]

Problem description
Several text refer to the Telco / Operator as the API provider, hence excluding Channel partners.
Also the definition of the term User seems strange - is it needed in addition to "End-User".

Expected action
Update these texts to generically refer to API provider. Pointers:
in the ICM document: CAMARA-API-access-and-user-consent.md

• change the occurence of "Telco Operator" in the template text line 360 to "API Provider". - NOTE: this impacts all API specifications referencing the template text. There is already an issue for that ICM text should not state that Telco Operators are the ones providing these APIs #190 which shall be done before chaning all the APIs.
• Also check and generalize as appropriate the other occurences of "Telco" or "operator" in the document (total 12 Telco and 81 operator).
• the definitions of User as an "operator subscriber" and "End-user" as the application user seems confusing. do we need to distinguish between an "End-User" of the application and a Telco subscriber "User" ? do we need User at all ?
• line 365: avoid the word "customer": API customer -> API consumer

Also generalize "Telco" occurence in the ICM document: https://github.com/camaraproject/IdentityAndConsentManagement/blob/main/documentation/CAMARA-Security-Interoperability.md